1. Field of the Invention
This invention is in the field of night vision devices which provide a visible image from low-level visible light or from light in the near-infrared (invisible) portion of the spectrum by use of an image intensifier tube. As used herein, the term "light" means electromagnetic radiation, regardless of whether or not this light is visible to the human eye.
Image intensifier tubes of such night vision devices generally include a photocathodes which is responsive to light in the infrared spectral range to release photoelectrons. Thus, the present invention is also in the field of such photocathodes. The photoelectrons released within such an image intensifier tube may be amplified or multiplied by conventional devices such as a microchannel plate or dynode to provide, for example, a current indicative of a light flux, or to produce an image of a light source or of an object illuminated with infrared light.
The present photocathode includes an active layer of indium gallium arsenide (InGaAs).
2. Related Technology
Night vision devices which use an image intensifier tube are well known. Generally, such devices include an objective lens by which light from a distant scene is received and focused upon a photocathode of the image intensifier tube. A power supply of the device provides appropriate voltage levels to various connections of the image intensifier tube so that this tube responsively provides a visible image. An eyepiece lens of the device provides the visible image to a user of the device.
Particularly, the image intensifier tube includes a photocathode responsive to light photons within a certain band of wavelengths to liberate photoelectrons. Because the photons are focused on the photocathode in a pattern replicating an image of a scene, the photoelectrons are liberated from the photocathode in shower having a pattern replicating this image of the scene. Within the image intensifier tube, the photoelectrons are moved by an applied electrostatic field to a microchannel plate, which includes a great multitude of microchannels. Each of the microchannels is effectively a dynode, which liberates secondary emission electrons in response to photoelectrons liberated at the photocathode. The shower of secondary emission electrons from the microchannel plate are moved to a phosphorescent screen which provides a visible image in yellow-green phosphorescent light.
Conventional photocathodes are disclosed in each of the following United States or foreign patents:
U.S. Pat. No. 3,814,996, issued Jun. 4, 1974, is believed to disclose a photocathode of an ternary alloy of indium, gallium, and arsenide of the formula In.sub.x Ga.sub.1-x As, in which "x" has a value of from 0.15 to 0.21.
U.S. Pat. No. 4,286,373, issued Sep. 1, 1981, is believed to disclose a photocathode of gallium arsenide at the photo-emitting layer, and is associated with a layer of gallium, aluminum, arsenide as a passivating layer.
U.S. Pat. No. 4,477,294, issued Oct. 16, 1984, is believed to relate to a photocathode of gallium arsenide as the photo-emitting layer, which is formed by hybrid epitaxy.
U.S. Pat. No. 4,498,225, issued Feb. 12, 1985, is thought to disclose a photocathode of gallium arsenide, formed on a glass substrate with intervening layers of gallium, aluminum, arsenide as passivation and anti-reflection layers.
U.S. Pat. No. 5,268,570, relates to a photocathode of indium gallium arsenide, grown on an aluminum indium arsenide window layer.
Similarly, U.S. Pat. No. 5,506,402, relates to a photocathode of indium gallium arsenide, grown on an aluminum gallium arsenide window layer.
British patent No. 1,478,453, issued Jun. 29, 1977, is believed to disclose a photocathode comprising (Ga.sub.1-x Al.sub.x).sub.1-z In.sub.z As, wherein (0.ltoreq.z&lt;y)
It appears that none of these conventional photocathodes are optimized to provide imaging at wavelengths above 950 nm. Such imaging is desired in order to allow active illumination of a scene with a laser. Conventional GaAs photocathodes have a long-wavelength cutoff of about 900 nm. The cutoff wavelength can be extended to the range of 900-1100 nm by using a ternary compound of indium, gallium, and arsenide. While the quantum efficiency of such photocathodes is less than conventional GaAs photocathodes, the greater photon availability under night-sight conditions compensates for this loss of efficiency. The photon activity of the night sky in the 800-900 nm band is five to seven times as great as in the visible region. Conventional photocathodes of the InGaAs type have a white-light response of about 300.mu./lm, with a radiant response at 1060 nm of about 0.025 mA/W.
A photocathode which achieves a white-light sensitivity of 500.mu./lm while maintaining a radiant response of greater than 30 mA/W to light of 980 nm wavelength is desirable.